1. Field of the Invention
The present invention relates to an active matrix substrate for a liquid crystal display and a method of fabricating an active matrix substrate.
2. Description of the Related Art
Active matrix substrates are widely used in liquid crystal displays and include interconnections. The interconnections include assemblies of scanning and signal lines. In order to create the interconnections, the fabrication of such active matrix substrates requires deposition of a mask each time upon application of photolithography and etching. Deposition of a mask requires time and cost. A need remains to minimize the number of steps of the mask deposition in a direction toward a cost and time reduction in fabricating an active matrix substrate. Various fabrication methods have been proposed, which use four or five masks. Examples of such fabrication methods using five masks are found in JP-A 9-171197 and JP-A 9-197433. An example using four masks is found in JP-A 2000-164886.
For reliable interconnections within a limited area, extensive efforts have been made on development of the material of scanning and signal lines. One example of such material is a pure aluminum (Al) because a thin film of pure aluminum is easy to form and it has a sufficiently low specific resistivity, which property scanning and signal lines should possess. Heat treatment, such as baking or annealing, after forming the scanning and signal lines out of the material is unavoidable. However, the pure aluminum has poor capacity to withstand heat, minute protrusions, called hillocks, are formed on the surface during such heat treatment. Although the mechanism of the formation of hillocks has not been clarified satisfactorily, stress migration, thermal migration, etc., play important roles. Such hillocks cause several problems, such as, short circuit among scanning and signal lines, and penetration of etchant through holes made in a dielectric layer and a protective layer due to the growth of hillocks.
To avoid the occurrence of hillocks, various aluminum alloys have been studied and proposed, which contains a small mass percent (wt %) high melting point metal or rare earth metal. Aluminum-neodymium (Al—Nd) alloy is one example.
The use of Al—Nd alloy as interconnections is known from JP-A 2000-275679, JP-A 2000-47240 and JP-A 2000-314897.
JP-A 2000-275679 shows a double-layered film including an under-layer of Al—Nd alloy and an over-layer of high melting point. It teaches wet etching the double-layered film to form gate electrodes of an active matrix substrate.
JP-A 2000-47240 shows a double-layered film including an under-layer of Al—Nd alloy that contains 1 wt % to 4.5 wt % Nd and an over-layer of high melting point metal. It teaches wet etching the double-layered film to form scanning lines or signal lines. The signal lines have a tapered cross sectional profile, the taper angle of which ranges from 40° to 55°. It also teaches forming such lines out of a triple-layered film. The triple-layered film includes an under-layer of high melting point metal, a middle-layer of Al—Nd alloy and an over-layer of high melting point metal. There is no specific description on the etching of such triple-layered film.
JP-A 2000-314897 shows scanning and signal lines, each coated with a layer of alumina. A double-layered film or a triple-layered film is wet etched to form such lines. The double-layered film includes an under-layer of high melting point metal and an over-layer of Al alloy. The triple-layered film includes an under-layer of high melting point metal, a middle-layer of Al alloy and an over-layer of high melting point metal. The high melting point metal is selected from a group consisting of pure Cr, Cr alloy, pure Mo and Mo alloy. The Al alloy contains 0.1 atomic % to 1.0 atomic % of at least one element selected from a group consisting of Ti, Ta, Nd, Y, La, Sm and Si.